1. Field of the Invention
The present invention relates to vertical cavity surface emitting lasers (VCSELs), and more particularly to VCSELs with mode control formed by selective patterning of upper mesa structures.
2. The Background Art
A typical VCSEL configuration includes an active region between two mirrors disposed one over another on the surface of the substrate wafer. An insulating region between the mirrors forces the current to flow through a small aperture, and the device lases perpendicular to the wafer surface (i.e., the “vertical” part of VCSEL). One type of VCSEL in particular, the proton VCSEL, wherein the insulating region is formed by a proton implantation, dominated the early commercial history of VCSELs. In the oxide-guided VCSEL, the insulating region is formed by partial oxidation of a thin, high aluminum-content layer within the structure of the mirror. This same oxidation process can be applied to other semiconductor structures, to produce both optoelectronic and purely electronic devices.
Vertical-cavity surface-emitting lasers (VCSELs) have become the laser technology of choice for transceivers used in Storage-Area Network (SAN) and Local Area Network (LAN) applications. There are two major technology platforms for manufacturing VCSELs. The difference in these platforms is based on the different techniques of current confinement, either by ion-implantation or confined by oxide layers. The two methods of forming a current confining structure in a VCSEL are ion implantation and selective oxidation. In the ion implantation technique, ions are implanted in a portion of the upper reflection layer so as to form a high resistance region, thereby confining the current flow to a defined region. In the selective oxidation technique, the peripheral region of a mesa structure is oxidized, thereby defining an aperture surrounded by a high resistance region.
More particularly, in the selective oxidation method, after depositing an AlGaAs layer on a lower portion of an upper reflector, which is to be a high-resistance region, the resultant structure is etched, resulting in individual VCSELs on a wafer. Next, the wafer is left in an oxidation atmosphere for a predetermined period of time, to allow diffusion of vapor into the peripheral portion of the AlAs layer. As a result, an oxide insulating layer is formed at the peripheral portion as the high-resistance region, which limits flow of current, thereby resulting in an aperture surrounded by the high-resistance region.
The oxidative diffusion rate in forming an aperture of a VCSEL is highly sensitive to the temperature of a furnace for use in the oxidative diffusion, oxidation time and the amount of oxygen supplied into the furnace. A variation in the diffusion rate is a serious problem in mass production that requires high repeatability, and in forming a particular size of the aperture.
The implanted VCSELs have been proven very reliable. However, the operating speed of the implanted VCSELs is usually limited for applications requiring less than 2 Gb/sec operating speed. Oxide VCSELs provide many superior properties of VCSEL performance including higher speed (demonstrated greater than 23 Gb/sec) and higher efficiency. However, the time in the field for SAN and LAN applications with oxide VCSELs has not been as long as the implanted VCSELs.
The electromagnetic wave propagation design of current commercially available 10 Gb/s VCSEL is single mode in the longitudinal or growth direction (z-axis) and multi-mode in the transverse or perpendicular to the growth direction (r-plane). Along the z-axis, the active semiconductor layer thicknesses are designed so that only a single optical mode couples to the laser gain peak. In the r-plane, the allowed transverse modes are determined by the size of the oxide aperture. Another mode determining characteristic is that looking outward radially from the center of the mesa, there is a gradual drop in the average refractive index of the layer of approximately 5% due the oxide aperture. This change in refractive index leads to index guiding of the transverse modes.
In the selective oxidation type VCSEL, if the diameter of a light emitting region (nearly corresponding to the diameter of a non-selective oxidation region) is enlarged for the purpose of increasing an output power, the VCSEL produces oscillations of various orders, that is, produces a so-called multimode oscillation. In the multimode oscillation, a spectral line width is made wide and the optical fiber has mode dispersion characteristics, so the attenuation of signal in the fiber is increased, or a mode state is made unstable and thus the main order of mode of the oscillation is easily varied by a change in the amount of current injected and a change in the environmental temperature. A dynamic change in the mode order is not preferable because it changes a coupling efficiency with the fiber.
To avoid the problem of mode instability due to multiple transverse modes, there have been a number or approaches suggested in the prior art. U.S. Pat. No. 6,990,128 describes a number of these approaches of controlling a transverse mode oscillation, and we reiterate such description here. The first approach to ensure oscillation only in the fundamental mode of the lowest order (0 order) is by making the diameter of the light emitting region smaller. However, when the diameter of the light emitting region of the VCSEL is reduced typically, to 4 um or less, (which is smaller than that of the high above-described proton injection type VCSEL) these VCSELs have a high element resistance and are thus unable to produce high output power. Making the transverse mode stable is an important requirement for preventing the signal from being attenuated when the VCSEL is optically coupled to the optical fiber. In addition, it is necessary to improve electric optical characteristics.
Among ideas for simultaneously realizing opposing goals of making the transverse mode stable, and reducing resistance and increasing output power in the selective oxidation type VCSEL having excellent luminous efficiency and high response performance, is a VCSEL having a structure disclosed in IEEE Photonics Technology Letters, Vo. 11, No. 12, page 1536-1538 (see FIG. 13). In this example, the diameter of the light emitting region is as large as 20 um but the inside of an electrode aperture emitting laser light is etched away to a depth of 40 nm except for a region of a radius of 7.75 um from the center of the aperture. Since the diameter of the light emitting region is as large as 20 um, in the case where there is no surface processing, the order of oscillation mode is varied in accordance with the amount of injection current and thus a far-field image is observed to vary; in contrast, a surface emitting semiconductor laser with a hole produces a fundamental mode up to an optical output of 0.7 mW but when current exceeding that level is injected, the mode splits to gradually widen the far-field image.
The purpose of the VCSEL described above is to improve the optical output power in the fundamental mode. However, the maximum optical output power of the surface emitting semiconductor laser with a hole is 10.4 mW, whereas the output power in the fundamental mode is only 0.7 mW. Taking into account that the maximum output power in the case where there is no surface processing is 17.9 mW, the prior art configuration described above clearly shows that it is very difficult to make the transverse mode stable and to produce a large optical output power at the same time.
In this respect, various other VCSEL structures for controlling the mode have been proposed. For example, U.S. Pat. No. 5,940,422 discloses a VCSEL in which a mode control is performed by forming two regions of different film thicknesses. In the '422 patent, only a region on which an additional film is deposited becomes a light emitting region. It is thought that the purpose of the invention is to artificially determine the position of a light emitting spot and not to determine the position by taking into consideration the specific oscillation to be produced in the VCSEL (for example, the oscillation mode of producing five light emitting spots, described as one preferred embodiment, does not exist in the natural world).
Further, U.S. Pat. No. 5,963,576 discloses a VCSEL having an annular waveguide. In particular, the invention provides a mode in which light emitting spots are arranged regularly in an annular region so as to produce a “super resolution spot” and not necessarily to deliberately produce a specific oscillation mode of a determined order.
IEEE Photonics Technology Letters, Vol. 9, No. 9, page 1193-1195, discloses a VCSEL having a configuration in which a circular cavity is formed on the top surface of a post by etching to locally vary a mirror reflectivity. The paper reports that the spectral line width of this device is reduced to a half of that of a device with no cavity to produce an effect of suppressing the mode. However, as the amount of current injected increases, an oscillation spectrum is observed to vary. This clearly shows that a specific oscillation mode is not always dominant, in other words, that the mode is not stable.
Further, Electronics Letters, Vol. 34, No. 7, page 681-682, (April 1998) proposes a VCSEL having a configuration in which a circular cavity is formed on the top surface of a post by etching and in which an annular light emitting region is formed on the outer peripheral portion of the cavity. It is clear from a near-field pattern that a very high order (larger than 30th order) mode is produced and at the same time that there are large variations in the intensity of light emitting spot. This shows that it is difficult to inject a uniform current into the annular region of an inside diameter as large as 30 um. Therefore, there is plenty of room for improvement of the VCSEL in order to obtain a stable high order mode oscillation for practical application.
As described above, as to the VCSEL expected as a light source for a multimode type optical fiber, the state of art in the VCSEL technology can not provide a device that satisfies a requirement of stabilizing a transverse mode and has high output power, low resistance, high efficiency and high speed response.
U.S. Pat. No. 6,990,128 discloses a method for fabricating a single mode VCSEL. However, the single mode it supports is a high order transverse mode instead of the fundamental mode.
U.S. Pat. No. 6,990,128 describes providing a resonator and discloses a structure with a first region in which a light emitting region is formed, an active layer, and a second reflection layer formed so as to sandwich the active layer between the first reflection layer and the second reflection layer, wherein the light emitting region includes a boundary region for suppressing the light emission of oscillation modes except for a specific oscillation mode; in particular a plurality of divided regions which are substantially divided by the boundary region to produce a light emitting spot corresponding to the specific oscillation mode.
The disadvantages of such a design are
1. Since all but a single mode is suppressed, the total power output of the device is low. FIG. 5 of U.S. Pat. No. 6,990,128 shows LI curves “with hole” and “without hole.” The total output power is reduced by almost 50%, and it appears that the LI curve rolls over at a lower drive current when the mode selection is employed. Accordingly, low output power limits the length of any optical link which uses such design.
2. As detailed in the patent (e.g. FIGS. 3A, 3B, 7A, 7B, 8A, 8B . . . ) a complex and precise pattern must be used in fabrication of the laser in order to select a single high-order mode. Additionally, the patent does not discuss how the alignment of the pattern to the oxide aperture effects the device performance.
3. Since the device is intentionally single mode when used with single mode fiber, such a single mode laser has the advantage of the elimination of modal dispersion. However, if a multimode fiber or a multimode waveguide is used, single mode lasers generally suffer significant jitter penalties due to mode mixing while propagating in the fiber. When using single mode lasers in multimode fiber, it is critical to precisely control the laser launch condition which adds cost, and complexity to the system. A second fundamental issue with single mode lasers is back reflection from the coupling optics into the laser cavity. Since a single mode is present the back reflection destabilizes the laser adding jitter to the signal. The industry standard solutions to this problem are either inserting an optical isolator between the laser and the coupling optics (which adds cost and complexity to the system), or restricting the laser power to a level where interference in the cavity is not a problem (however, due to the power restrictions, this limits the applications where this device can be employed).
A more detailed analysis and description of the transverse modes in a VCSEL would be useful at this point. The transverse modes can be grouped into two classes: oxide aperture center modes (ACM) and oxide aperture edge modes (AEM). Due to optical scattering by the oxidized layer, the ACM's will always have lower intrinsic loss than the AEM's. As a result, the ACM's have lower threshold gains, and they will lase before the AEM's, and dominate laser emission near threshold. However, excepting the case of a transparent contact covering the laser emission aperture, the injected current will always have a radial component moving from the outside of the oxide aperture toward the aperture center. Because of this radial current injection, well above threshold the AEM modes will dominate the laser emission. Above threshold, the carrier lifetime drops dramatically due to stimulated emission. Consequently, the carrier diffusion length drops and they are no longer able to reach the center of the aperture.
The difficulty in achieving narrow spectral width is caused by tradeoffs inherent in choosing the size of the oxide aperture. Smaller apertures, reduce the number of allowed transverse modes, but have a number of problems related to device reliability: First, the device resistance is inversely proportional to the square of the aperture diameter. From this perspective the minimum aperture size is set by impedance matching to the driver. Second, the ESD damage threshold is also inversely proportional to the square of the aperture diameter. Lower ESD thresholds add cost and complexity to the manufacturing process and increase the risk of field failures. Third, the wear out reliability is proportional to the square of the current density. At constant current, the wear out lifetime is inversely proportional to the aperture diameter to the fourth power. Fourth, the thermal impedance is inversely proportional to aperture diameter. Smaller devices have higher junction temperatures and hence reduced wear out lifetimes. Fifth, smaller apertures require a higher fraction of oxidized AlGaAs, which increases the mechanical strain in the laser.
Another issue in prior art VCSELs is the sensitivity of the spectral width to drive current and ambient temperature. These effects are caused by both the competition between ACMs and AEMs, and the number of transverse modes that are present due to the size of the oxide aperture. As drive current increases, more of the higher loss AEMs reach threshold. Hence, the SW of the laser increases with drive current. At low temperature, the overall loss in the laser decreases, and more of the AEMs reach threshold. Hence, the SW also increases with decreasing temperature.
Still another issue is the problem of jitter and undershoot caused by mode competition in modulating a multimode VCSEL caused by the alternating dominance of the ACM and AEM modes as the laser is modulated. Compared to the AEM modes, the ACM modes respond slower to current modulation. First, the carriers have to diffuse farther to reach the aperture center. Second, the AEM modes have additional loss due to scattering by the oxide aperture. The net result is reduction in the optical lifetime which allows the optical modes to better track the drive current as the laser is modulated.
Prior to the present invention, there has not been a commercially practical oxidation type VCSEL with narrow spectral width suitable for use within an optical mouse.